Deep Mixing and Metallicity: Carbon Depletion in Globular Cluster Giants

TL;DR

This study investigates how the efficiency of deep mixing in globular cluster red giants varies with metallicity by measuring carbon depletion rates across a range of stellar metallicities.

Contribution

It provides observational evidence that carbon depletion rates increase with decreasing metallicity, supporting theoretical models of deep mixing in stellar evolution.

Findings

Carbon depletion rate is twice as high at [Fe/H]=-2.3 compared to [Fe/H]=-1.3.

Deep mixing efficiency correlates with stellar metallicity.

Results support theoretical predictions of metallicity-dependent mixing processes.

Abstract

We present the results of an observational study of the efficiency of deep mixing in globular cluster red giants as a function of stellar metallicity. We determine [C/Fe] abundances based on low-resolution spectra taken with the Kast spectrograph on the 3m Shane telescope at Lick Observatory. Spectra centered on the 4300 Angstrom CH absorption band were taken for 42 bright red giants in 11 Galactic globular clusters ranging in metallicity from M92 ([Fe/H]=-2.29) to NGC 6712 ([Fe/H]=-1.01). Carbon abundances were derived by comparing values of the CH bandstrength index S2(CH) measured from the data with values measured from a large grid of SSG synthetic spectra. Present-day abundances are combined with theoretical calculations of the time since the onset of mixing, which is also a function of stellar metallicity, to calculate the carbon depletion rate across our metallicity range. We find that the carbon depletion rate is twice as high at a metallicity of [Fe/H]=-2.3 than at [Fe/H]=-1.3, which is a result qualitatively predicted by some theoretical explanations of the deep mixing process.